This invention relates generally to a spinal implant assembly for implantation into the intervertebral space between adjacent vertebral bones to simultaneously provide stabilization and continued flexibility and proper anatomical motion, and more specifically to such a device that utilizes a wave washer force restoring element.
The bones and connective tissue of an adult human spinal column consists of more than 20 discrete bones coupled sequentially to one another by a tri-joint complex that consists of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than 20 bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine, up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are the thoracic vertebrae, and connect to the lower spine comprising the 5 lumbar vertebrae. The base of the spine is the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic spine, which are in turn smaller than those of the lumbar region. The sacral region connects laterally to the pelvis. While the sacral region is an integral part of the spine, for the purposes of fusion surgeries and for this disclosure, the word spine shall refer only to the cervical, thoracic, and lumbar regions.
The spinal column is highly complex in that it includes these more than 20 bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these complications, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction.
Genetic or developmental irregularities, trauma, chronic stress, tumors, and degenerative wear are a few of the causes that can result in spinal pathologies for which surgical intervention may be necessary. A variety of systems have been disclosed in the art that achieve immobilization and/or fusion of adjacent bones by implanting artificial assemblies in or on the spinal column. The region of the back that needs to be immobilized, as well as the individual variations in anatomy, determine the appropriate surgical protocol and implantation assembly. With respect to the failure of the intervertebral disc, the interbody fusion cage has generated substantial interest because it can be implanted laparoscopically into the anterior of the spine, thus reducing operating room time, patient recovery time, and scarification.
Referring now to FIGS. 7 and 8, in which a side perspective view of an intervertebral body cage and an anterior perspective view of a post implantation spinal column are shown, respectively, a more complete description of these devices of the prior art is herein provided. These cages 10 generally comprise tubular metal body 12 having an external surface threading 14. They are inserted transverse to the axis of the spine 16, into preformed cylindrical holes at the junction of adjacent vertebral bodies (in FIG. 8 the pair of cages 10 are inserted between the fifth lumbar vertebra (L5) and the top of the sacrum (S1)). Two cages 10 are generally inserted side by side with the external threading 14 tapping into the lower surface of the vertebral bone above (L5), and the upper surface of the vertebral bone (S1) below. The cages 10 include holes 18 through which the adjacent bones are to grow. Additional materials, for example autogenous bone graft materials, may be inserted into the hollow interior 20 of the cage 10 to incite or accelerate the growth of the bone into the cage. End caps (not shown) are often utilized to hold the bone graft material within the cage 10.
These cages of the prior art have enjoyed medical success in promoting fusion and grossly approximating proper disc height. It is, however, important to note that the fusion of the adjacent bones is an incomplete solution to the underlying pathology as it does not cure the ailment, but rather simply masks the pathology under a stabilizing bridge of bone. This bone fusion limits the overall flexibility of the spinal column and artificially constrains the normal motion of the patient. This constraint can cause collateral injury to the patient""s spine as additional stresses of motion, normally borne by the now-fused joint, are transferred onto the nearby facet joints and intervertebral discs. It would therefore, be a considerable advance in the art to provide an implant assembly which does not promote fusion, but, rather, which nearly completely mimics the biomechanical action of the natural disc cartilage, thereby permitting continued normal motion and stress distribution.
It is, therefore, an object of the invention to provide an intervertebral spacer that stabilizes the spine without promoting a bone fusion across the intervertebral space.
It is further an object of the invention to provide an implant device that stabilizes the spine while still permitting normal motion.
It is further an object of the invention to provide a device for implantation into the intervertebral space that does not promote the abnormal distribution of biomechanical stresses on the patient""s spine.
It is further an object of the invention to provide an artificial disc that has an plate attachment device (for attaching the plates of the artificial disc to the vertebral bones between which the disc is implanted) with superior gripping and holding strength upon initial implantation and thereafter.
It is further an object of the invention to provide an artificial disc plate attachment device that deflects during insertion of the artificial disc between vertebral bodies.
It is further an object of the invention to provide an artificial disc plate attachment device that conforms to the concave surface of a vertebral body.
It is further an object of the invention to provide an artificial disc plate attachment device that does not restrict the angle at which the artificial disc can be implanted.
It is further an object of the invention to provide an artificial disc that supports tension loads.
It is further an object of the invention to provide an artificial disc that provides a centroid of motion centrally located within the intervertebral space.
Other objects of the invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter.
The preceding objects are achieved by the invention, which is an artificial intervertebral disc or intervertebral spacer device comprising a pair of support members (e.g., spaced apart plates), each with an exterior surface. Because the artificial disc is to be positioned between the facing surfaces of adjacent vertebral bodies, the plates are arranged in a substantially parallel planar alignment (or slightly offset relative to one another in accordance with proper lordotic angulation) with the exterior surfaces facing away from one another. The plates are to mate with the vertebral bodies so as to not rotate relative thereto, but rather to permit the spinal segments to axially compress and bend relative to one another in manners that mimic the natural motion of the spinal segment. This natural motion is permitted by the performance of a spring disposed between the secured plates, and the securing of the plates to the vertebral bone is achieved through the use of a vertebral body contact element including, for example, a convex mesh attached to the exterior surface of each plate. Each convex mesh is secured at its perimeter, by laser welds, to the exterior surface of the respective plate. While domed in its initial undeflected conformation, the mesh deflects as necessary during insertion of the artificial disc between vertebral bodies, and, once the artificial disc is seated between the vertebral bodies, the mesh deforms as necessary under anatomical loads to reshape itself to the concave surface of the vertebral endplate. Thus, the wire mesh is deformably reshapeable under anatomical loads such that it conformably deflects against the concave surface to securably engage the vertebral body endplate. Stated alternatively, because the wire mesh is convexly shaped and is secured at its perimeter to the plate, the wire mesh is biased away from the plate but moveable toward the plate (under a load overcoming the bias; such a load is present, for example, as an anatomical load in the intervertebral space) so that it will securably engage the vertebral body endplate when disposed in the intervertebral space. This affords the plate having the mesh substantially superior gripping and holding strength upon initial implantation, as compared with other artificial disc products. The convex mesh further provides an osteoconductive surface through which the bone may ultimately grow. The mesh preferably is comprised of titanium, but can also be formed from other metals and/or non-metals. Inasmuch as the mesh is domed, it does not restrict the angle at which the artificial disc can be implanted. It should be understood that while the flexible dome is described herein preferably as a wire mesh, other meshed or solid flexible elements can also be used, including flexible elements comprises of non-metals and/or other metals. Further, the flexibility, deflectability and/or deformability need not be provided by a flexible material, but can additionally or alternatively be provided mechanically or by other means.
To enhance the securing of the plates to the vertebral bones, each plate further comprises at least a lateral porous ring (which may be, for example, a sprayed deposition layer, or an adhesive applied beaded metal layer, or another suitable porous coating known in the art). This porous ring permits the long-term ingrowth of vertebral bone into the plate, thus permanently securing the prosthesis within the intervertebral space. The porous layer may extend beneath the domed mesh as well, but is more importantly applied to the lateral rim of the exterior surface of the plate that seats directly against the vertebral body.
The spring disposed between the plates provides a strong restoring force when a compressive load is applied to the plates, and also permits rotation and angulation of the two plates relative to one another. While a wide variety of embodiments are contemplated, a preferred spring includes a wave washer utilized as the restoring force providing element. In general, a wave washer is one of the strongest configurations for a spring, and is highly suitable for use as a force restoring providing subassembly for use in an intervertebral spacer element that must endure considerable cyclical loading in an active human adult. A compressive load applied to the plates causes a corresponding compression of the wave washer, which is turn causes a restoring force to be applied to the plates. The wave washer deflects appropriately under the load, only to spring back to its undeflected shape upon the unloading.
In particular, in order for the overall device to mimic the mechanical flexibility of the natural disc, it is desirable that the spring provide restoring forces that (1) are directed outward against the opposing plates, when a compressive load is applied to the plates; (2) that permit lateral bending and flexion and extension bending of the plates relative to parallel; (3) that do not permit lateral translation of the plates relative to one another during such bending; and (4) that do not substantially interfere with the rotation of the opposing plates relative to one another. The wave washers disclosed herein provide such functionality.
The wave washers of the invention have a circumferential extent surrounding a central bore. The circumferential extent is concentrically wavy, such that the extent undulates along a concentric path around the central bore to form radially extending valleys and peaks, while preferably maintaining a constant radius. Stated equivalently with regard to the most basic wave washer embodiments of the invention, which resemble traditional wave washers, the wave washer is a simple round washer having a circumferential extent that comprises a flat round ring, except that while maintaining a constant curvature of radius in the plane normally defined by the washer, the circumferential extent rises and falls in a wave-like curve. Whereas a standard (non-wave) washer has a circumferential extent that is confined to the x-y plane, the wave washer has a circumferential extent that extends in the x-y plane but undulates in the z-axis. Herein, the top and bottom of a wave washer shall be defined as the planes defined by the lowest and highest points of the undulations, respectively. In some embodiments, the circumferential extent is continuous (i.e., has no slots). In other embodiments, the circumferential extent has at least one radially extending slot. In still other embodiments, the circumferential extent has at least one radially extending and spiraling slot. The thickness of the circumferential extent; the frequency, amplitude, and configuration of the waves; and/or the number and configuration of the slots can be varied to accommodate any desired application, inasmuch as varying the dimensions will affect the behavior of the wave washer in expansion and retraction.
The restoring force of a wave washer is proportional to the elastic properties of the material. As a compressive load is applied to the wave washer, the forces are directed down onto the peaks and up against the valleys. A significant fraction of these forces are immediately translated into hoop stresses that apply stresses directly toward radially expanding the wave washer. This hoop stress is also counterbalanced by the material strength of the wave washer. The strain of the material causes a deflection in the height of the washer and a slight radial expansion. The slots in the slotted embodiments permit the compressive load that is applied to the wave washer down onto the peaks and up against the valleys to cause the wave washer to deflect such that the slots close. Thus, a difference between a slotted washer and a continuous washer is that the continuous washer responds to a compressive load by primarily deflecting radially (with a very high stress to deflection ratio), whereas the slotted washer primarily deflects circumferentially, closing the slots (which is characteristic of a much lower stress to deflection ratio). Stated equivalently, a wave washer responds to a compressive load by deflecting compressively, and either radially or circumferentially. With at least one slot formed in the washer, it expands and retracts far more elastically than a continuous washer. It should be understood that wave washers other than those shown are contemplated by the invention, including but not limited to wave washers having a circumferential extent that does not have a uniformly wide radius.
As described above, the most basic wave washer of the invention has a circumferential extent that defines a circumference of 360 degrees (or less if the wave washer includes a radial slot that passes completely through the circumferential extent). Another wave washer embodiment of the invention, instead of being ring-shaped, is spiral-shaped, having a circumferential extent that defines a circumference of more than 360 degrees, and preferably more than 720 degrees, or more depending on the specific anatomical needs of the patient. The undulations of the wave washer in the z-axis may be such that the arches are aligned, or misaligned. In yet another wave washer embodiment of the invention, instead of using a spiral-shaped wave washer, multiple concentric ring-shaped wave washers can be used in conjunction with one another to achieve a similar functional result.
Still another wave washer embodiment of the invention is also spiral-shaped, but has an amplitude of the undulations that decreases in the radial direction. The wave washer thereby takes on the edge-on appearance of a spiral galaxy, having a thicker central portion, and a flatter edge. In this case, the restoring force varies according to the number of spirals of the washer and according to the number of spirals that are engaged (more radially distal spirals are engaged as the deflection of the washer increases). More specifically, as a compressive load is applied by a pair of plates against the top and bottom of a spiral wave washer, the forces are first directed against the peaks of the undulating waves at the center of the spiral, and are then increasingly directed against the peaks of the outer portions of the spiral. In a further wave washer embodiment of the invention, instead of using a spiral-shaped wave washer with radially decreasing undulation amplitudes, multiple concentric ring-shaped wave washers can be used in conjunction with one another, positioned so that those with smaller undulation amplitudes are more radially distant from the center of the grouped washers, to achieve a similar functional result. It should be understood that in either of these types of embodiments, the wave washers can be formed such that the undulation amplitudes increase, rather than decrease, with their radial distance from the center of the washer, or such that the undulation amplitudes vary in size either randomly or according to other patterns.
With regard to additional wave washer embodiments, changing the configuration of the circumferential extent in other ways modifies the magnitude of the compressive load support and restoring force provided by the wave washer. For clarity and conciseness, the other circumferential extent configurations discussed herein are illustrated with regard to wave washers having circumferential extents that are ring-shaped (as opposed to spiral-shaped) and thicker compared to the wave washer embodiments summarized above (as those summarized embodiments are illustrated), however it should be understood that the additional circumferential extent variations discussed herein can be applied individually or in various combinations to the spiral-shaped, concentric, and/or radially varying undulation amplitude configurations, without departing from the scope of the invention.
For example, a variety of circumferential extents are illustrated and discussed herein. In some embodiments, the circumferential extent is generally planar (e.g., the extent extends in a plane and all of the waves undulate perpendicular to that plane). In other embodiments, the circumferential extent is generally conical (e.g., the extent extends to define a conical surface concentric with the central bore and the waves undulate perpendicular to that surface at their respective positions on the surface) and radially straight, such that the height of the wave washer is linearly related to the radial width of the circumferential extent. In still other embodiments, the circumferential extent is generally semispherical (e.g., the extent extends to define a semispherical surface concentric with the central bore and the waves undulate perpendicular to that surface at their respective positions on the surface) and radially bowed, such that the height of the wave washer is not linearly related to the radial width of the circumferential extent (but rather the wave washer may, for example, be parabolic in shape). In still other embodiments, the circumferential extent extends radially downwardly from the central bore. In still other embodiments, the circumferential extent is doubled, with a lower portion extending radially downwardly from the central bore and an upper portion extending radially upwardly from the central bore. By changing the circumferential extent from a generally planar configuration to a generally conical or generally semispherical configuration, the resting height of the washer is increased and the radial expansion potential of the washer is increased while the structural integrity of the washer is enhanced. The shape and direction of the circumferential extent can be varied to accommodate desired applications, inasmuch as varying the dimensions will affect the behavior of the wave washer in expansion and retraction.
Also, for example, additional configurations of the circumferential extent are possible, and are illustrated and discussed herein, to affect the behavior of the wave washer in expansion and retraction. In some embodiments, in addition to the concentric waviness common to all of the wave washer embodiments, the circumferential extent has at least one concentric or radial characteristic that alters the performance of the wave washer in expansion and/or retraction. More specifically, in some embodiments, the circumferential extent is not only concentrically wavy, but is also radially wavy. In other embodiments, the circumferential extent is radially thinning (the portion of the extent near the central bore is thicker than the portion of the extent near the outer edge of the washer). In still other embodiments, the circumferential extent is radially thickening (the portion of the extent near the central bore is thinner than the portion of the extent near the outer edge of the washer). In still other embodiments, the circumferential extent is concentrically grooved, having grooves that are similarly dimensioned to one another regardless of their relative radial distance from the central bore, or grooves that vary in dimension from one another depending on their relative radial distance from the central bore. These alterations, depending on the configuration, cause certain portions (e.g., grooved, thinner, or more wavy portions) of the circumferential extent to expand more readily than other portions (e.g., non-grooved, thicker or less wavy portions).
It should be noted that with regard to the waves of the wave washers of the invention, one or both of the depth and the width of each wave can be (1) decreasing along the length of the wave from the outer edge of the washer toward the central bore, (2) increasing along the length of the wave from the outer edge of the washer toward the central bore, (3) uniform along the length of the wave from the outer edge of the washer toward the central bore, or (4) varied along the length of each wave from the outer edge of the washer toward the central bore, either randomly or according to a pattern. Moreover, it can be the case that each wave is not formed similarly to one or more other waves, but rather one or more waves are formed in any of the above-mentioned fashions, while one or more other waves are formed in another of the above-mentioned fashions or other fashions. It should be clear that any wave pattern can be implemented without departing from the scope of the invention. By making the wave pattern non-uniform, certain portions of the circumferential extent give more readily than other portions, and therefore the behavior of the wave washer in expansion and retraction can be modified and/or controlled.
For disposing the wave washer (whichever wave washer embodiment is chosen for the clinical application) between the plates, each wave washer embodiment has at least one feature suitable for this purpose, and the plates of the artificial disc comprise cooperating features suitable for this purpose. With regard to the wave washer features, each wave washer embodiment has a central bore and at least one end that expands and retracts as described above. The central bore of some wave washer embodiments forms a curvate socket on a narrow end of the wave washer, for coupling with a ball-shaped protuberance on a plate as described below.
With regard to the structure and coupling features of the plates, three plate embodiments are illustrated and described herein, although other suitable plate embodiments can be used with the invention. Each of the three plate embodiments has the above described convex mesh on its outwardly facing surface, although other vertebral body attachment devices and mechanisms can be used without departing from the scope of the invention. Each of the three plate embodiments has a different inwardly facing surface from the other two plate embodiments. The first plate embodiment has a flat inwardly facing surface that accepts a fastener (e.g., a screw, plug, dowel or rivet; a rivet is used herein as an example) for rotatably securing thereto a narrow end of a wave washer having a circumferential extent that is generally conical or generally semispherical, and/or that accepts a flanged (and preferably rotatable) fastener (e.g., a screw, plug, dowel, rivet, or spoked post; a rotatable spoked post is used herein as an example) for securing thereto a wave washer having a circumferential extent that is generally planar. The second plate embodiment has a circular recess on its inwardly facing surface, for rotationally housing an end of a wave washer and allowing the end to expand in unrestricted fashion when the wave washer is compressed. The third plate embodiment has a semispherical (e.g., ball-shaped) protuberance on its inwardly facing surface, for rotatably and angulatably holding a narrow end of a wave washer, which narrow end includes a curvate socket as described below.
The semispherical protuberance has an axial bore that receives a deflection preventing element (e.g., a rivet, plug, dowel, or screw; a rivet is used herein as an example). Prior to the insertion of the rivet, the ball-shaped protuberance can deflect radially inward (so that the ball-shaped protuberance contracts). The insertion of the rivet eliminates the capacity for this deflection. The curvate socket, having a substantially constant radius of curvature that is also substantially equivalent to the radius of the ball-shaped protuberance, accommodates the ball-shaped protuberance for free rotation and angulation once the ball-shaped protuberance is disposed in the curvate socket, but in the ball-shaped protuberance""s undeflected state, the ball-shaped protuberance cannot fit through the opening leading to the curvate socket. Therefore, the deflectability of the ball-shaped protuberance, prior to the insertion of the rivet, permits the ball-shaped protuberance to be inserted into the curvate socket. Subsequent introduction of the rivet into the axial bore of the ball-shaped protuberance prevents the ball-shaped protuberance from deflecting, and thus prevents the ball-shaped protuberance from escaping the socket. Thereby, the ball-shaped protuberance can be secured in the curvate socket so that it rotates and angulates therein through a range of angles, thus permitting the plates to rotate and angulate relative to one another through a corresponding range of angles equivalent to the fraction of normal human spine rotation and angulation (to mimic normal disc rotation and angulation).
With the three plate embodiments, the various wave washer embodiments, and the several manners in which they may be coupled together, it is possible to assemble a variety of artificial disc embodiments. Many examples are described herein, although many permutations that are contemplated and encompassed by the invention are not specifically identified herein, but are readily identifiable with an understanding of the invention as described. For example, any of the wave washers can be disposed between circular recesses of opposing plates. Also for example, all wave washers having a curvate socket can have the curvate socket coupled with a ball-shaped protuberance on a plate. Also for example, all wave washers having a simple bore (i.e., without a curvate socket) can have the simple bore coupled with a flat inwardly facing surface of a plate using a fastener (e.g., a rotatable spoked post or a screw or a rivet). Also for example, each wave washer having a wide end (e.g., wave washers having a circumferential extent that is generally conical or generally semispherical) can be disposed with its wide end in a circular recess of a plate, and a retaining element (e.g., a shield) can be secured over the wave washer after it has been placed in the circular recess to prevent the wave washer from escaping the recess when a tension load is applied to the plates.
Each assembly enjoys spring-like performance with respect to axial compressive loads, as well as long cycle life to mimic the axial biomechanical performance of the normal human intervertebral disc. The wave washer expands radially and/or circumferentially under a compressive load, only to spring back into its undeflected shape when it is unloaded. As the wave washer compresses and decompresses, the walls of the circular recess of the second plate embodiment maintain the end of the wave washer within a prescribed boundary on the inwardly facing surface of the plate. Certain assemblies withstand tension loads on the outwardly facing surfaces, because (in embodiments having a generally conical or generally semispherical extent) the shield retains the wide end in the circular recess and because (in embodiments using the ball-shaped protuberance) the rivet in the axial bore prevents the ball-shaped protuberance from deflecting, thus preventing it from exiting the curvate socket and because (in embodiments in which the narrow end of a wave washer is secured by a rivet or a rotatable spoked post), the flanged portion of the rivet (or the spokes of the post) prevents the wave washer from escaping the circular recess. Accordingly, in such embodiments, once the plates are secured to the vertebral bones, the assembly will not come apart when a normally experienced tension load is applied to the spine, similar to the tension-bearing integrity of a healthy natural intervertebral disc.
Assemblies having the ball-and-socket joint also provide a centroid of motion centrally located within the intervertebral space, because the plates are made rotatable and angulatable relative to one another by the ball-shaped protuberance being rotatably and angulatably coupled in the curvate socket. The centroid of motion remains in the ball-shaped protuberance, and thus remains centrally located between the vertebral bodies, similar to the centroid of motion in a healthy natural intervertebral disc.